Acoustic correction apparatus

ABSTRACT

An acoustic correction apparatus processes a pair of left and right input signals to compensate for spatial distortion as a function of frequency when the input signals are reproduced through speakers in a sound system. The sound-energy of the left and right input signals is separated and corrected in a first low-frequency range and a second high-frequency range. The resultant signals are recombined to create image-corrected audio signals having a desired sound-pressure response when reproduced by the speakers in the sound system. The desired sound-pressure response creates an apparent sound image location with respect to a listener. The image-corrected signals may then be spatially enhanced to broaden the apparent sound image.

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/169,352, filed on Oct. 9, 1998, which is a continuation ofU.S. patent application Ser. No. 08/508,593, filed on Jul. 28, 1995, nowU.S. Pat. No. 5,850,453, the entirety of which are hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to audio enhancement systems,and especially those systems and methods designed to improve the realismof stereo sound reproduction. More particularly, this invention relatesto apparatus for overcoming the acoustic deficiencies of a sound systemas perceived by a listener which can result when speakers within thesound system are not ideally positioned.

[0004] 2. Description of the Related Art

[0005] In a sound reproduction environment various factors may serve todegrade the quality of reproduced sound as perceived by a listener. Suchfactors distinguish the sound reproduction from that of an originalsound stage. One such factor is the location of speakers in a soundstage which, if inappropriately placed, may lead to a distortedsound-pressure response over the audible frequency spectrum. Theplacement of speakers also affects the perceived width of a soundstage.For example, speakers act as point sources of sound limiting theirability to reproduce reverberant sounds that are easily perceived in alive sound stage. In fact, the perceived sound stage width of many audioreproduction systems is limited to the distance separating a pair ofspeakers when placed in front of a listener. Another factor degradingthe quality of reproduced sound may result from microphones which recordsound differently from the way the human hearing system perceives sound.In an attempt to overcome the factors which degrade the quality ofreproduced sound, countless efforts have been expended to alter thecharacteristics of a sound reproduction environment to mimic that heardby a listener in a live sound stage.

[0006] Some efforts at stereo image enhancement have focused on theacoustic abilities and limitations of the human ear. The human ear'sauditory response is sensitive to sound intensity, phase differencesbetween certain sounds, the frequency of the sound itself, and thedirection from which sound emanates. Despite the complexity of the humanauditory system, the frequency response of the human ear is relativelyconstant from person to person.

[0007] When sound waves having a constant sound pressure level acrossall frequencies are directed at a listener from a single location, thehuman ear will react differently to the individual frequency componentsof the sound. For example, when sound of equal sound pressure isdirected towards a listener from in front of the listener, the pressurelevel created within the listener's ear by a sound of 1000 hertz will bedifferent from that of 2000 hertz.

[0008] In addition to frequency sensitivity, the human auditory systemreacts differently to sounds impinging upon the ear from various angles.Specifically, the sound pressure level within the human ear will varywith the direction of sound. The shape of the outer ear, or pinna, andthe inner ear canal are largely responsible for the frequency contouringof sounds as a function of direction.

[0009] The human auditory response is sensitive to both azimuth andelevation changes of a sound's origin. This is particularly true forcomplex sound signals, i.e., those having multiple frequencycomponents,. and for higher frequency components in general. Thevariance in sound pressure within the ear is interpreted by the brain toprovide indications of a sound's origin. When a recorded sound isreproduced, the directional cues to the sound's origin, as interpretedby the ear from sound pressure information, will thus be dependent uponthe actual location of speakers that reproduce the sound.

[0010] A constant sound pressure level, i.e., a “flat” sound pressureversus frequency response, can be obtained at the ears of a listenerfrom loudspeakers positioned directly in front of the listener. Such aresponse is often desirable to achieve a realistic sound image. However,the quality of a set of speakers may be less than ideal, and they maynot be placed in the most acoustically-desirable location. Both suchfactors often lead to disrupted sound pressure characteristics. Soundsystems of the prior art have disclosed methods to “correct” the soundpressure emanating from speakers to create a spatially correct responsethereby improving the resulting sound image.

[0011] To achieve a more spatially correct response for a given soundsystem, it is known to select and apply head-related-transfer-functions(HRTFs) to an audio signal. HRTFs are based on the acoustics of thehuman hearing system. Application of an HRTF is used to adjust theamplitudes of portions of the audio signal to compensate for spatialdistortion. HRTF-based principles may also be used to relocate a stereoimage from non-optimally placed loudspeakers.

[0012] The efforts made in the prior art to correct acousticdeficiencies within an audio reproduction system have often focused onthe deficiencies present in automobile sound systems. One such attemptis disclosed in both U.S. Pat. No. 4,648,117 issued to Kunugi, et al.,and U.S. Pat. No. 4,622,691 issued to Tokumo, et al. In the disclosuresof Kunugi and Tokumo, a system for correcting sound absorption levelsand for avoiding sound wave interference is described for use within avehicle.. The disclosed system includes a sound-pressure correctingcircuit and a signal-delay circuit for achieving the desired frequencyresponse. The sound-pressure correction is achieved by a high-frequencyboost of the sound signal applied in three stages. The first stage is ahigh-frequency correction for the average sound absorption factor of avehicle, the second high-frequency correction stage is dependent uponthe sound absorption factor of a specific vehicle, and the thirdhigh-frequency correction factor is dependent upon the number ofpassengers seated within the vehicle.

[0013] In U.S. Pat. No. 5,146,507 issued to Satoh et al., an audioreproduction system control device is disclosed for correcting thefrequency response of a given reproduction environment to match that ofa standard frequency response characteristic. The system in Satohprovides a correction parameter for sound signals directed to frontleft, front right, rear left and rear right speakers of a sound field,such as in an automobile. Prestored acoustic characteristics relating tofrequency and reflection are utilized to adapt the audio reproductioncontrol device to a variety of sound environments.

[0014] Another system designed to modify a frequency responsecharacteristic within an automobile is disclosed in U.S. Pat. No.4,888,809 issued to Knibbeler. The system of Knibbeler attempts tocreate a flat frequency response at two separate non-coincidentlistening positions, such as the front and rear positions in anautomobile passenger compartment, by adjusting a pair of filter units.Each of the filter units receives an input signal and affects an outputsignal delivered to a corresponding sound transducer.

[0015] Still other patents disclose sound systems, which alter an audiosignal to equalize the frequency response. Such patents include U.S.Pat. No. 5,371,799 issued to Lowe, et al., U.S. Pat. No. 5,325,435issued to Date, et al., U.S. Pat. No. 5,228,085 issued to Aylward, U.S.Pat. No. 5,033,092 issued to Sadaie, U.S. Pat. No. 4,393,270 issued tovan den Berg, and U.S. Pat. No. 4,329,544 issued to Yamada.

SUMMARY OF THE INVENTION

[0016] Despite the contributions from the prior art, there exists a needfor an image correction apparatus which can easily be adapted to avariety of sound reproduction environments which have distorted spatialcharacteristics. There is also a need for such an image correctionsystem which operates in conjunction with an image enhancement apparatusto spatially enhance the corrected stereo image.

[0017] The acoustic correction apparatus as disclosed herein, and theassociated methods of operation, provide a sophisticated and effectivesystem for improving a sound image in an imperfect reproductionenvironment.

[0018] To achieve an improved stereo image, an image correction devicedivides an input signal into first and second frequency ranges whichcollectively contain substantially all of the audio frequency spectrum.The frequency response characteristics of the input signal within thefirst and second frequency ranges are separately corrected and combinedto create an output signal having a relatively flat frequency-responsecharacteristic with respect to a listener. The level of frequencycorrection, i.e., sound-energy correction, is dependent upon thereproduction environment and tailored to overcome the acousticlimitations of such an environment. The design of the acousticcorrection apparatus allows for easy and independent correction of theinput signal within individual frequency ranges to achieve aspatially-corrected and relocated sound image.

[0019] Within an audio reproduction environment, speakers may be placedat a location remote from a listener's ears thereby adversely affectinga sound image perceived by the listener. For example, within anautomobile, speakers for producing low, mid, and high range audiosignals may be positioned in door panels below the listener's ears. Theacoustic correction apparatus of the present invention relocates thesound image to an apparent position near the listener's ear level.

[0020] In some audio reproduction environments, the high-frequencytransducers, or tweeters, are placed at locations remote from mid-rangeor low-frequency transducers, i.e., mid-range or woofer speakers. In anautomobile, mid-range speakers are often placed in door panels orsimilar locations located near the legs or feet of a listener. Tweeters,however, may be positioned at a height near or above the listener's earlevel to avoid interference or absorption by surrounding objects. Thesmall size of tweeters allows for such remote placement within avehicle. When tweeters are placed near a listener's ear, the soundpressure level at the listener's ears among the high-frequency rangesmay be greater than the corresponding low-frequency ranges. Accordingly,the acoustic correction apparatus is designed so that correction of thehigher frequency components may be either positive or negative. That is,the higher frequency components may be either boosted or attenuated,relative to a lower frequency component, to compensate for remoteplacement of the tweeters.

[0021] Through application of the acoustic correction apparatus, astereo image generated from playback of an audio signal may be spatiallycorrected to convey a perceived source of origin having a verticaland/or horizontal position distinct from the position of the speakers.The exact source of origin perceived by a listener will depend on thelevel of spatial correction. In the context of an automobile, theacoustic correction apparatus disclosed herein may be used, inconnection with door-mounted speakers, to achieve a substantially flatfrequency response at an occupant's ear. Such a response will create anapparent stereo image positioned in front of the listener at approximateear level.

[0022] Once a perceived sound origin is obtained through correction ofspatial distortion, the corrected audio signal may be enhanced toprovide an expanded stereo image. In accordance with a preferredembodiment, stereo image enhancement of a relocated audio image takesinto account acoustic principles of human hearing to envelop thelistener in a realistic sound stage. In those sound reproductionenvironments where a listening position is relatively fixed, such as theinterior of an automobile, the amount of stereo image enhancementapplied to the audio signal is partially determined by the actualposition of the speakers with respect to the listener.

[0023] According to one preferred aspect of the invention, an audiocorrection device is adaptable to an automotive sound system of avehicle for spatially enhancing a stereo image projected by theautomotive sound system with respect to a listener situated within adriver seat of the vehicle. The vehicle also has a forward-sectionpassenger seat and the automotive sound system comprises a pair ofspeakers mounted within a respective driver-side door and passenger-sidedoor of the vehicle wherein the speakers are positioned beneath a pairof ears of the listener.

[0024] The preferred audio correction device comprises a stereo imagecorrection circuit connected to the automotive sound system forreceiving a stereo sound signal, the stereo sound signal exhibitingaudio distortion with respect to the listener when played through thespeakers, the image correction circuit modifying components of thestereo sound signal to generate a corrected stereo sound signal, thecorrected stereo sound signal compensating for the audio distortion toprovide an apparent sound image for the listener when the correctedstereo sound signal is played through the speakers.

[0025] In addition, the preferred audio correction device furthercomprises a stereo image enhancement circuit receiving the correctedstereo sound signal for broadening the apparent sound image, the stereoimage enhancement circuit modifying the stereo sound signal to generatea spatially enhanced sound signal for playback through the speakers, theimage enhancement circuit comprising a means for isolating the stereoinformational content of the stereo sound signal, an equalizer forapplying a level of amplitude boost to said stereo informational contentas a function of frequency wherein said level of boost is characterizedby a maximum gain below 200 hertz and characterized by a minimum gainbetween 1 kHz and 5 kHz, and means for combining said stereoinformational content with said stereo signal to create said spatiallyenhanced sound signal.

[0026] In another embodiment, the stereo image correction circuitdivides an audible frequency spectrum into a low frequency range and ahigh frequency range relative to the low frequency range, the imagecorrection circuit modifying components of the stereo sound signalwithin the low frequency range independently of components within thehigh frequency range.

[0027] In another embodiment, the distortion results from placement ofthe speakers within the door whereby the speakers are pointed towardsrespective sides of the listener, the speakers characterized by an angleof sound dispersion such that the listener's ears are positionedsubstantially outside of the angle of sound dispersion.

[0028] In another embodiment, playback of the spatially enhanced soundsignal through the speakers has an apparent effect of rotating theapparent sound image towards the listener, and wherein the minimum gainof the stereo informational content signal is a function of the positionof the speaker system with respect to the listener. In anotherembodiment, the distortion results from sound-absorption characteristicsof an interior of the vehicle.

[0029] According to another aspect of the invention, an audioenhancement apparatus is operative upon left and right stereo inputsignals provided by a stereo reproduction device for playback through aspeaker system having a fixed location within an audio reproductionenvironment, the enhancement apparatus modifying the stereo inputsignals to obtain an improved stereo image by compensating for acousticlimitations created when the input signals are reproduced by the speakersystem within the audio reproduction environment. The audio enhancementapparatus comprising a stereo image correction circuit receiving theleft and right stereo input signals and modifying the input signals togenerate energy-corrected left and right stereo signals, theenergy-corrected left and right signals creating a corrected spatialresponse when played through the speaker system and heard by a listenerin the audio reproduction environment, the corrected spatial responsecreating an apparent sound image with respect to the listener to obtaina realistic and redirected sound experience for the listener.

[0030] This embodiment of the audio enhancement apparatus furthercomprising a stereo image enhancement circuit receiving theenergy-corrected left and right stereo signals and generating enhancedleft and right stereo signals for enhancing the apparent sound image toprovide an improved sound image perceived by the listener when theenhanced left and right stereo signals are reproduced through thespeaker system and wherein the energy-corrected left and right signalsare characterized by a first difference-signal component representingthe difference between the energy-corrected left and right signals, andthe enhanced left and right stereo signals are characterized by a seconddifference-signal component representing the difference between theenhanced left and right signals, the second difference-signal componentselectively equalized with respect to the first difference-signalcomponent.

[0031] In another embodiment, the acoustic limitations are a function ofthe fixed location of the speaker system with respect to the listener.In another embodiment, the acoustic limitations are inherentcharacteristics of the speaker system. In yet another embodiment, theapparent sound image is defined by an azimuth and elevation with respectto the listener different from that of the speaker system.

[0032] In another embodiment, the acoustic limitations are a function ofthe fixed location of the speaker system with respect to the listenerand a function of sound absorption characteristics of the audioreproduction environment. In yet another embodiment, the correctedspatial response is characterized by sound-pressure energy levels whichare substantially constant across all audible frequencies above 100 Hzwith respect to the listener.

[0033] In another embodiment, the stereo image correction circuitcomprises a first correction circuit for modifying components of theleft and right input signals within a first frequency range to create afirst corrected stereo signal component, a second correction circuit forseparately modifying components of the left and right input signalswithin a second frequency range to create a second corrected stereosignal component, and means for combining the first and second correctedstereo signal components to generate the energy-corrected left and rightsignals.

[0034] In another embodiment, the means for combining also combines arespective one of the input signals with the first and second correctedstereo signal components to generate the energy-corrected left and rightsignals. In yet another embodiment, the first corrected stereo signalcomponent comprises signals having frequencies between approximately 100Hz to 1 kHz, and the second corrected stereo signal component comprisessignals having frequencies between approximately 1 kHz to 10 kHz.

[0035] In another embodiment, the second corrected stereo signalcomponent is attenuated by the energy-correction circuit. In yet anotherembodiment, the second correction circuit boosts the input signalcomponents within the second frequency range to generate the secondcorrected stereo signal component, the means for combining furthercomprising a switch having a first position and a second position,wherein the second corrected stereo signal component is added to thefirst corrected stereo signal component by the means for combining whenthe switch is in the first position, and the second corrected stereosignal component is subtracted from the first corrected stereo signalcomponent when the switch is in the second position.

[0036] In another embodiment, the stereo image enhancement circuitcomprises an equalizer for altering a frequency response of the firstdifference signal to create the second difference signal by applying aperspective equalization curve to the first difference signal, theperspective equalization curve characterized by a maximum-gain turningpoint occurring at a maximum-gain frequency within a first frequencyrange of approximately 100 to 200 hertz and the curve characterized by aminimum-gain turning point occurring at a minimum-gain frequency withina second frequency range of approximately 1680 to 5000 hertz.

[0037] In another embodiment, the maximum gain is within a range ofapproximately 10 to 15 dB, and the minimum gain is within a range ofapproximately 0 to 10 dB. In yet another embodiment, the maximum gain,the maximum-gain frequency, the minimum gain, and the minimum-gainfrequency are dependent upon the fixed location of the speaker systemwith respect to the listener. In an additional embodiment, theperspective equalization curve is a function of an angle created between(1) the path of direct-field sound emanating from the speaker system andimpinging upon a proximate ear of the listener, and (2) a plane parallelto the listener's forward line-of-sight.

[0038] In another embodiment, the audio enhancement apparatus isimplemented in digital format by a digital signal processor. In yetanother embodiment, the audio enhancement apparatus is implemented usingdiscrete circuit components. In additional embodiment, the left andright stereo input signals are synthetically generated from a monophonicaudio signal source. In an additional embodiment, the left and rightstereo input signals are part of an audio-visual composite signal.

[0039] In another embodiment, the audio enhancement apparatus isconstructed as a digital and analog hybrid circuit. In yet anotherembodiment, the audio enhancement system is contained within asemiconductor substrate. In an additional embodiment, the audioenhancement system is contained within a multi-chip module.

[0040] In another embodiment, the audio reproduction environment is theinterior of an automobile having first and second door panels positionedon opposite sides of a driver of the automobile and wherein the speakersystem comprises a first speaker positioned within the first door paneland a second speaker positioned within the second door panel. In yetanother embodiment, the audio reproduction environment is associatedwith an electronic keyboard apparatus having a keyboard and wherein thespeaker system comprises first and second speakers connected to theelectronic keyboard apparatus, the first and second speakers placedbeneath the keyboard.

[0041] According to another aspect of the invention, a stereoenhancement device receives or inputs a pair of stereophonic left andright audio signals and provides processed left and right audio signalsto a speaker system for reproduction of a sound image corresponding tothe processed signals. This embodiment of the stereo enhancement devicecomprises means for selectively altering the amplitude levels of theleft and right audio signals to create corrected left and right audiosignals, the corrected left and right audio signals conveying aperceived source of origin for the sound image with respect to alistener when the corrected signals are played through the speakersystem, the perceived source of origin distinct from an actual source oforigin for the sound image, and means for enhancing the corrected leftand right audio signals to emphasize reverberant sound energy present inthe corrected left and right audio signals, the means for enhancingproducing the processed left and right audio signals.

[0042] In another embodiment, the means for enhancing the correctedsignals amplifies selected frequency components of a difference signalby predetermined amounts, the difference signal representing the amountof stereo information present in the corrected left and right audiosignals, and the predetermined amounts determined as a function of theactual source of origin for the sound image. In yet another embodiment,playback of the left and right audio signals through the speaker systemgenerates a first frequency-dependent sound pressure response withrespect to a listener, and playback of the left and right audio signalsthrough a speaker system located at the perceived source of origingenerates a second frequency-dependent sound pressure response withrespect to the listener, the corrected left and right audio signalsgenerating the second frequency-dependent sound pressure response withrespect to the listener when the corrected left and right audio signalsare reproduced by the speaker system.

[0043] In another embodiment, the means for selectively altering furthercomprises means for dividing the stereophonic audio signals intolow-frequency components and high-frequency components, means forequalizing the low and high frequency components to generatelow-frequency and high-frequency energy-corrected audio signals, andmeans for combining the low and high-frequency energy-corrected audiosignals to generate the corrected left and right audio signals.

[0044] In another embodiment, the low-frequency components are containedwithin a frequency range of approximately 100 to 1000 Hz, and thehigh-frequency components are contained within a frequency range ofapproximately 1000 to 10,000 Hz. In yet another embodiment, thelow-frequency components correspond to a first frequency range of thestereophonic audio signals and the high-frequency components correspondto a second frequency range of the stereophonic audio signals, thelow-frequency components boosted over the first frequency range and thehigh-frequency components attenuated over the second frequency range.

[0045] In another embodiment, the means for enhancing comprises meansfor generating a sum signal representing the sum of the corrected leftaudio signal and the corrected right audio signal, means for generatinga difference signal representing the difference between the correctedleft audio signal and the corrected right audio signal, means forboosting components of the difference signal within a first and secondrange of frequencies relative to components of the difference signalwithin a third range of frequencies to create a processed differencesignal, the third range of frequencies greater than the first range offrequencies and less than the second range of frequencies, and means forcombining the sum signal and the processed difference signal to createthe processed left and right audio signals.

[0046] In another embodiment, the difference signal has a minimum-gainturning point occurring at a minimum-gain frequency within the thirdrange of frequencies, the minimum-gain turning point determined as afunction of the actual source of origin for the sound image. In yetanother embodiment, the components of the difference signal within thefirst, second, and third range of frequencies are all amplified by themeans for boosting.

[0047] According to another aspect of the invention, a spatialenhancement apparatus redirects and enhances a stereophonic imageemanating from a speaker system located within an audio reproductionenvironment. In this aspect of the invention, the spatial enhancementapparatus comprises an acoustic-image correcting circuit receiving anaudio input signal and producing a corrected audio signal, the audioinput signal creating a first sound-pressure response with respect to alistener when played through the speaker system, and the corrected audiosignal creating a second sound-pressure response when played through thespeaker system, the second sound-pressure response generating anapparent stereo image corresponding to an apparent location of thespeaker system with respect to the listener, and an acoustic-imageenhancement circuit receiving the corrected audio signal and providingan enhanced audio signal for reproduction through the speaker system,the enhanced audio signal equalized with respect to the corrected audiosignal to broaden the apparent stereo image.

[0048] In another embodiment, the corrected audio signal is astereophonic signal comprising a difference signal representing theamount of stereo information present in the corrected audio signal, theacoustic-image enhancement circuit equalizing the difference signal toemphasize reverberant sound energy in the corrected audio signal forbroadening the apparent stereo image.

[0049] In another embodiment, the corrected audio signal is astereophonic signal comprising a difference signal representing theamount of stereo information present in the corrected audio signal, theacoustic-image enhancement circuit equalizing the difference signalaccording to a perspective level of equalization to create a processeddifference signal, the perspective level of equalization varying withrespect to frequency of the difference signal and characterized by amaximum gain occurring at a maximum-gain frequency within a firstfrequency range of approximately 100 to 200 hertz and a minimum gainoccurring at a minimum-gain frequency within a second frequency range ofapproximately 1680 to 5000 Hz, the level of equalization decreasingbelow the first frequency range and above the first frequency rangetowards the minimum-gain frequency, the level of equalization furtherincreasing above the minimum-gain frequency. In yet another embodiment,the maximum gain and the minimum gain are a function of an actuallocation of the speaker system relative to a listener within the audioreproduction environment.

[0050] In another embodiment, the level of equalization of thedifference signal is further characterized by bass attenuation of thedifference signal relative to the maximum gain, the bass attenuationoccurring below the maximum-gain frequency and the bass attenuationincreasing with a reduction in difference-signal frequency. In yetanother embodiment, the maximum gain and the minimum gain are fixed atpreset gain levels, the maximum gain and the minimum gain dependent uponthe angle of incidence of direct-field sound emanating from an actuallocation of the speaker system and reaching an ear of the listener.

[0051] In another embodiment, the acoustic-image correcting circuitcomprises a first filter receiving the audio input signal and providinga first filtered output signal, the first filter having afrequency-response characteristic comprising a first transition band,the audio input signal having amplitude levels modified throughout thefirst transition band as a function of frequency, a second audio filterreceiving the audio input signal and providing a second filtered outputsignal, the second audio filter having a frequency-responsecharacteristic comprising a second transition band, the audio inputsignal having amplitude levels modified throughout the second transitionband as a function of frequency, and an amplifier for boosting theamplitude levels of the first and second filtered output signals, andfor combining the first and second filtered output signals with theaudio input signal to generate the corrected audio signal, the correctedaudio signal creating the apparent stereo image when reproduced throughthe speaker system.

[0052] In another embodiment, the audio input signal comprises a leftinput signal and a right input signal, and the acoustic-image correctingcircuit comprises a first energy-correction device receiving the leftinput signal for processing the left input signal to generate acorrected left audio signal, the first energy-correction devicecomprising, a low-frequency correction circuit receiving the left inputsignal and providing a corrected low-frequency left signal, thelow-frequency correction circuit boosting amplitude components of theleft input signal within a first frequency range, a high-frequencycorrection circuit receiving the left input signal and providing acorrected high-frequency left signal, the high-frequency correctioncircuit adjusting amplitude components of the left input signal within asecond frequency range, means for combining the corrected low andhigh-frequency left signals to create the corrected left audio signal, asecond energy-correction device receiving the right input signal togenerate a corrected right audio signal.

[0053] The second energy-correction device further comprising alow-frequency correction circuit receiving the right input signal andproviding a corrected low-frequency right signal, the low-frequencycorrection circuit boosting amplitude components of the right inputsignal within the first frequency range, a high-frequency correctioncircuit receiving the right input signal and providing a correctedhigh-frequency right signal, the high-frequency correction circuitadjusting amplitude components of the right input signal within thesecond frequency range; and means for combining the corrected low andhigh-frequency right signals to create the corrected right audio signal.

[0054] In another embodiment, the audio reproduction environment is theinterior of an automobile, the automobile having a dashboard and theapparent stereo image emanating from the direction of the dashboardtowards the listener. In yet another embodiment, the audio reproductionenvironment is an outdoor area and wherein the listener may be situatedat a plurality of locations within the audio reproduction environment.

[0055] In another embodiment, the acoustic-image enhancement circuitcomprises a first summing network inputting corrected left and rightaudio signals supplied by the acoustic-image correcting circuit, thefirst summing network generating a difference signal and a sum signal,the difference signal representing the amount of stereo informationpresent in the corrected left and right audio signals, an equalizerconnected to the first summing network, the equalizer modifying thefrequency response of the difference signal to create a processeddifference signal having a level of equalization varying with respect tofrequency of the processed difference signal.

[0056] The level of equalization in this embodiment characterized by amaximum gain occurring at a maximum-gain frequency between approximately100 to 200 Hz and a minimum gain occurring at a minimum-gain frequencybetween approximately 1680 to 5000 Hz, mid-range attenuation of thedifference signal relative to the maximum gain, the mid-rangeattenuation occurring above the maximum-gain frequency and increasingwith a corresponding increase in difference-signal frequency up to theminimum-gain frequency with the mid-range attenuation decreasing abovethe minimum-gain frequency with an increase in difference-signalfrequency.

[0057] The acoustic-image enhancement circuit in this embodiment furthercomprising a signal mixer receiving the processed difference signal andcombining the processed difference signal with the sum signal and thecorrected left audio signal to create an enhanced left output signal forreproduction by the speaker system, the signal mixer also combining theprocessed difference signal with the sum signal and the corrected rightaudio signal to create an enhanced right output signal for reproductionby the speaker system.

[0058] According to another aspect of the invention, an acoustic energycorrection device for modifies the spectral density of a stereo signalto overcome acoustic deficiencies of a speaker system when the stereosignal is reproduced through the speaker system. In this aspect of theinvention, the acoustic energy correction device comprises acompensating circuit receiving the stereo signal for adjustingamplitudes of the stereo signal to obtain a desired acoustic spatialresponse with respect to a listener when the stereo signal is playedthrough the speaker system, the compensating circuit comprising, a firstcorrection circuit receiving the stereo signal and boosting the stereosignal as a first function of frequency over a first frequency range tocreate a first corrected stereo signal, a second correction circuitreceiving the stereo signal and adjusting the stereo signal as a secondfunction of frequency over a second frequency range to create a secondcorrected stereo signal, wherein the first function of frequency isindependent of the second function of frequency, and means for combiningthe first and second corrected stereo signals to create anenergy-corrected output signal.

[0059] In another embodiment, the first frequency range comprisesaudible frequencies below approximately 1000 Hz, and the secondfrequency range comprises audible frequencies above approximately 1000Hz. In yet another embodiment, the stereo signal is also combined withthe first and second corrected stereo signals by the means forcombining. In an additional embodiment, the level of boost applied bythe first correction circuit increases with a corresponding increase infrequency.

[0060] In another embodiment, the second correction circuit boosts thestereo signal within the second frequency range, the boost having alevel increasing with a corresponding increase in frequency. In yetanother embodiment, the second correction circuit attenuates the stereosignal within the second frequency range.

[0061] In an additional embodiment, the acoustic energy correctiondevice further includes an electronic switch receiving the secondcorrected stereo signal and providing an output connected to the meansfor combining, the electronic switch having a first position and asecond position, the first and second corrected stereo signals added bythe means for combining when the switch is in the first position, andthe second corrected stereo signal subtracted from the first correctedstereo signal when the switch is in the second position.

[0062] According to another aspect of the invention, an electronicdevice creates an apparent sound image from sound signals reproducedthrough an acoustic transducer. In this aspect of the invention, theelectronic device comprises a first filter receiving the sound signalsand providing a first filtered output signal, the first filter having afrequency-response characteristic comprising a first pass band and afirst transition band, the sound signals having amplitude levelsmodified throughout the first transition band as a function of frequencyand having a substantially uniform level of modification within thefirst pass band, a second audio filter receiving the sound signals andproviding a second filtered output signal, the second audio filterhaving a frequency-response characteristic comprising a second pass bandand a second transition band, the sound signals having amplitude levelsmodified throughout the second transition band as a function offrequency and having a substantially uniform level of modificationwithin the second pass band, and amplification means for boosting theamplitude levels of the first and second filtered output signals, andfor combining the first and second filtered output signals with thesound signals to generate energy-corrected sound signals, the energycorrected-sound signals creating the apparent sound image whenreproduced through the acoustic transducer.

[0063] In another embodiment, the second filtered output signal isinverted by the amplification means when combined with the firstfiltered output signal and the sound signals. In yet another embodiment,the first and second audio filters are high-pass filters, the firsttransition band having a frequency range between approximately 100 Hzand 1000 Hz, and the second transition band having a frequency rangebetween approximately 1000 Hz and 10 kHz.

[0064] In additional embodiment, the electronic device further comprisesmeans for spatially enhancing the energy-corrected sound signals, theenergy-corrected sound signals comprising a left energy-corrected signaland a right energy-corrected signal. The means for spatially enhancingcomprises means for generating a sum signal representing the sum of theenergy-corrected left and right signals, means for generating adifference signal representing the difference between theenergy-corrected left signal and the energy-corrected right signal, anequalizer for boosting components of the difference signal within afirst and second range of frequencies relative to components of thedifference signal within a third range of frequencies to create aprocessed difference signal, the third range of frequencies greater thanthe first range of frequencies and less than the second range offrequencies, and means for combining the sum signal and the processeddifference signal to create spatially-enhanced left and right outputsignals.

[0065] In another embodiment, the sound signals comprise left and rightsignals, and the amplification means comprises a first amplifier forboosting left signal components of the filtered output signals, and asecond amplifier for boosting right signal components of the filteredoutput signals, the first and second amplifiers applying a varying levelof boost to the filtered output signals, the level of boost adjustablethrough first and second ganged variable resistors, the first and secondganged variable resistors transferring the filtered output signals tothe amplification means.

[0066] An additional aspect of the invention also provides a method ofprocessing an audio signal to compensate for distortion of sound-energywhen the audio signal is reproduced through speakers in a sound system.The method of this aspect of the invention comprises the followingsteps: (a) creating a first filtered audio signal, the first filteredaudio signal characterized by a first transition band and a first passband of frequencies, (b) creating a second filtered audio signal, thesecond filtered audio signal characterized by a second transition bandand a second pass band of frequencies, (c) boosting amplitude componentsof the first filtered audio signal as a function of frequency within thefirst transition band, (d) boosting amplitude components of the firstfiltered audio signal by a fixed amount within the first pass band, (e)modifying amplitude components of the second filtered audio signal as afunction of frequency within the second transition band, (f) modifyingamplitude components of the second filtered audio signal by a fixedamount within the second pass band, (g) combining the boosted firstfiltered audio signal and the modified second filtered audio signal tocreate a spatially corrected audio signal to create a corrected soundimage when the spatially corrected audio signal is reproduced throughthe speakers, and (h) spatially enhancing the corrected audio signal tobroaden the corrected sound image.

[0067] In another embodiment, the first transition band is a frequencyrange below approximately 1000 hertz, the first pass band comprisesfrequencies above approximately 1000 hertz, the second transition bandis a frequency range of approximately 1000 hertz to 10,000 hertz, andthe second pass band comprises frequencies above approximately 10,000hertz. In another embodiment, the step of spatially enhancing thecorrected audio signal comprises the following steps: (a) generating adifference signal representing the stereo information content of thespatially corrected audio signal, and (b) altering the difference signalto create a processed difference signal by applying a perspectiveequalization curve to the difference signal, the perspectiveequalization curve characterized by a maximum-gain turning pointoccurring at a maximum-gain frequency within a first frequency range ofapproximately 100 to 200 hertz and the curve characterized by aminimum-gain turning point occurring at a minimum-gain frequency withina second frequency range of approximately 1680 to 5000 hertz.

[0068] Another aspect of the present invention provides a method ofcompensating for acoustic spatial distortion perceived by a listenerwithin an audio reproduction environment when an audio signal isreproduced through a speaker system also positioned within thereproduction environment. The method comprises the following steps: (a)separating the audio signal into a first group of signal componentswithin a first frequency range and a second group of components within asecond frequency range, the first group of signal components containedwithin a first frequency range below approximately 1000 Hertz and thesecond group of signal components contained within a second frequencyrange above approximately 1000 Hertz, (b) boosting amplitude levels ofthe first group of signal components as a function of frequency over thefirst frequency range to create a first modified group of signalcomponents, (c) adjusting amplitude levels of the second group ofcomponents as a function of frequency over the second frequency range tocreate a second modified group of signal components, and (d) combiningthe first modified group of signal components with the second modifiedgroup of signal components to create an energy-corrected audio outputsignal.

[0069] In another embodiment, the second modified group of signalcomponents are attenuated with respect to the second group of signalcomponents. In yet another embodiment, the method further comprises thestep of boosting amplitude levels of the audio signal within the secondfrequency range by a substantially fixed amount over the secondfrequency range, the fixed amount corresponding to a maximum level ofboost applied to the first group of signal components.

BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The above and other aspects, features, and advantages of thepresent invention will be more apparent from the following particulardescription thereof presented in conjunction with the followingdrawings, wherein:

[0071]FIG. 1 is a schematic block diagram of a stereo image correctioncircuit operatively connected to a stereo enhancement circuit forcreating a realistic stereo image from a pair of input stereo signals.

[0072]FIG. 2 is a diagram of a vehicle, as viewed from the side, showingthe placement of speakers within the interior portion of the vehicle.

[0073]FIG. 3 is a diagram of the vehicle in FIG. 2, as viewed from thetop, showing the placement of speakers within the interior portion ofthe vehicle.

[0074]FIG. 4A is a graphical representation of a desired sound-pressureversus frequency characteristic for an audio reproduction system.

[0075]FIG. 4B is a graphical representation of a sound-pressure versusfrequency characteristic corresponding to a first audio reproductionenvironment.

[0076]FIG. 4C is a graphical representation of a sound pressure versusfrequency characteristic corresponding to a second audio reproductionenvironment.

[0077]FIG. 4D is a graphical representation of a sound pressure versusfrequency characteristic corresponding to a third audio reproductionenvironment.

[0078]FIG. 5 is a schematic block diagram of an energy-correctioncircuit operatively connected to a stereo image enhancement circuit forcreating a realistic stereo image from a pair of input stereo signals.

[0079]FIG. 6A is a graphical representation of the various levels ofsignal modification provided by a low-frequency correction circuit inaccordance with a preferred embodiment.

[0080]FIG. 6B is a graphical representation of the various levels ofsignal modification provided by a high-frequency correction circuit forboosting high-frequency components of an audio signal in accordance witha preferred embodiment.

[0081]FIG. 6C is a graphical representation of the various levels ofsignal modification provided by a high-frequency correction circuit forattenuating high-frequency components of an audio signal in accordancewith a preferred embodiment.

[0082]FIG. 6D is a graphical representation of a compositeenergy-correction curve depicting the possible ranges of sound-pressurecorrection for relocating a stereo image.

[0083]FIG. 7 is a graphical representation of various levels ofequalization applied to an audio difference signal to achieve varyingamounts of stereo image enhancement.

[0084]FIG. 8A is a diagram depicting the perceived and actual origins ofsounds heard by a listener from speakers placed at a first location.

[0085]FIG. 8B is a diagram depicting the perceived and actual origins ofsounds heard by a listener from speakers placed at a second location.

[0086]FIG. 9 is a schematic diagram of an energy-correction circuit foraltering the sound pressure level of an audio signal across a broadfrequency range.

[0087]FIG. 10 is a schematic diagram of a stereo image enhancementcircuit for use in conjunction with the energy-correction circuit ofFIG. 9.

[0088]FIG. 11 is a schematic diagram of an alternative embodiment of astereo image enhancement circuit for use in conjunction with theenergy-correction circuit of FIG. 9.

[0089]FIG. 12 is a schematic diagram of a bass-boost circuit for use inan alternative embodiment of the present invention.

[0090]FIG. 13 is a diagram of a first alternative audio reproductionenvironment suitable for application of the present invention.

[0091]FIG. 14 is a perspective view of a second alternative audioreproduction environment suitable for application of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0092] Referring initially to FIG. 1, a block diagram showing apreferred embodiment of the present invention is shown. Specifically, anacoustic correction apparatus 20 comprises a stereo image correctioncircuit 22 coupled to a stereo image enhancement circuit 24. The imagecorrection circuit 22 inputs a left stereo signal 26 and a right stereosignal 28. An image-corrected left stereo signal, L_(c), and rightstereo signal, R_(c), are transmitted to the stereo image enhancementdevice 24 along paths 27 and 29, respectively. The stereo imageenhancement circuit 24 processes the signals, L_(c) and R_(c), andprovides a left output signal 30 and a right output signal 32. Theoutput signals 30 and 32 may in turn be connected to some other form ofsignal conditioning circuit, or they may be connected directly tospeakers (not shown).

[0093] In a preferred embodiment of the present invention, the stereoimage correction circuit 22 and the stereo image enhancement circuit 24will operate in conjunction to overcome acoustic deficiencies of a soundreproduction environment. Such sound reproduction environments may be aslarge as a theater complex or as small as a portable electronickeyboard. One such environment where the advantages of the presentinvention are particularly effective is the interior of an automobile.

[0094] Referring now to FIG. 2, a vehicle 40 having an audioreproduction system is depicted to display, in a side-cutaway view, theinterior seating arrangements and speaker placements of the vehicle 40.Similarly, in FIG. 3, the same speaker placements for the audioreproduction system are shown from a top view. The interior of thevehicle 40 can be divided into a forward section 42 and a rear section44. The sound system of the vehicle 40 includes a pair of door-mountedspeakers 46 mounted near the legs or feet of a forward-section occupant48. Similarly, the rear section 44 of the vehicle 40 also includes apair of door-mounted speakers 50. The locations of the speaker pairs 46and 50 within the door panels is a popular choice of automobilemanufacturers. In some vehicles, however, the door-mounted speakers 46and 50 may be replaced by or supplemented with speakers 52 mounted onthe rear parcel tray 54.

[0095] In addition, some automobile stereo systems have separate speakerlocations to reproduce mid-range or lower-frequency sounds at differentlocations than sounds of higher frequencies. The vehicle 40 of FIG. 2demonstrates such a stereo system. Specifically, a pair ofhigh-frequency speakers 56, i.e., tweeters, are mounted above theoccupant 48. The mounting position of the speakers 56 is often intendedto avoid distortion and interference of the high-frequency sounds whichmay occur from objects within the vehicle 40. The location of thespeakers 56 is possible due to the small size of tweeters.

[0096] Apart from the speaker positions shown in FIG. 2, there arevarious other possible locations where speakers may be placed in anautomobile by either a manufacturer or by an aftermarket specialist. Forexample, speakers may be placed within the dashboard 55 or in otherareas of the door panels 58. Regardless of the type of automobile stereosystem, or the placement of speakers therein, it is desirable to achievea frontal stereo image from the stereo system with respect to a listenersituated within the vehicle.

[0097]FIG. 4A depicts a graphical representation of a desired frequencyresponse characteristic, appearing at the outer ears of a listener,within an audio reproduction environment. The curve 60 is a function ofsound pressure level (SPL), measured in decibels, versus frequency. Ascan be seen in FIG. 4A, the sound pressure level is relatively constantfor all audible frequencies. The curve 60 can be achieved fromreproduction of pink noise through a pair of ideal speakers placeddirectly in front of a listener at approximately ear level. Pink noiserefers to sound delivered over the audio frequency spectrum having equalenergy per octave. In practice, the flat frequency response of the curve60 may fluctuate in response to inherent acoustic limitations of speakersystems.

[0098] The curve 60 represents the sound pressure levels that existbefore processing by the ear of a listener. Referring back to FIGS. 2and 3, the flat frequency response represented by the curve 60 isconsistent with sound emanating towards the occupant 48, from thelocation of the dashboard 55, in the direction A as shown. The human earprocesses such sound, as represented by the curve 60, by applying itsown auditory response to the sound signals. This human auditory responseis dictated by the outer pinna and the interior canal portions of theear.

[0099] Unfortunately, the frequency response characteristics of manyautomotive sound reproduction systems do not provide the desiredcharacteristic shown in FIG. 4A. On the contrary, speakers may be placedin acoustically-undesirable locations to accommodate other ergonomicrequirements. Referring again to FIG. 2, the door-mounted speakers 46and 50 are positioned at a convenient and unobtrusive location. However,in such a position, sound emanating from the speakers 46 and 50 may bespectrally distorted by the mere placement of the speakers 46 and 50with respect to the occupant 48. Moreover, the interior surroundings ofthe automobile 40, such as the legs of the occupant 48 and theautomobile seats 45, may lead to absorption, or amplitude distortion, ofthe resulting sound signals. Such absorption, which is often prevalentamong higher frequencies, has been the focus of some audio enhancementssystems of the prior art.

[0100] As a result of both spectral and amplitude distortion, a stereoimage perceived by the occupant 48 is spatially distorted providing anundesirable listening experience. FIGS. 4B-4D graphically depict levelsof spatial distortion for various automotive sound reproduction systems.The distortion characteristics depicted in FIGS. 4B-4D represent soundpressure levels, measured in decibels, which are present near the earsof a listener.

[0101] The frequency response curve 64 of FIG. 4B has a decreasingsound-pressure level at frequencies above approximately 100 Hz. Thecurve 64 represents a possible sound pressure characteristic generatedfrom speakers, containing both woofers and tweeters, which are mountedin an automobile below a listener. For example, assuming the speakers 46of FIG. 2 contain tweeters, an audio signal played through only suchspeakers 46 might exhibit the response of FIG. 4B. Assuming the soundpressure response of FIG. 4B is obtained in the automobile of FIG. 2,the occupant 48 will localize a resulting sound image in the lowerportion of the forward section 42.

[0102] The particular slope associated with the decreasing curve 46 willlikely vary, and may not be entirely linear, depending on theautomobile's interior, the quality of the speakers, and the exactpositioning of the speakers within the door panels 58. For example, aleather or vinyl interior will be more reflective of audio signals,particularly at higher frequencies, than a cloth interior. The level ofspectral distortion will vary significantly as speakers are placedfurther from, and positioned away from, a listener.

[0103]FIG. 4C is a graphical representation of a sound-pressure versusfrequency characteristic 68 wherein a first frequency range of audiosignals are spectrally distorted, but a higher frequency range of thesignals are not distorted. The characteristic curve 68 may be achievedfrom a speaker arrangement having low to mid-frequency speakers placedbelow a listener and high-frequency speakers positioned near, or at alistener's ear level. Referring again to FIG. 2, such low tomid-frequency speakers would correspond to the speakers 46, while suchhigh-frequency speakers (not shown) would be placed somewhere on thedashboard 55. With this arrangement, the frequency response curve 68 hasa maximum amplitude level at approximately 100 Hz which decreases as afunction of frequency up to approximately 1000 Hz. At 1000 Hz, the curve68 again increases up to the maximum amplitude level. The increase insound pressure level above 1000 Hz is a direct result of tweeters placedin an unobstructed position in front of the vehicle's occupant 48. Thesound image resulting from the characteristic curve 68 will have alow-frequency component positioned below the occupant 48 of FIG. 2, anda high-frequency component positioned near the occupant's ear level.

[0104]FIG. 4D is a graphical representation of a sound-pressure versusfrequency characteristic 70 having a reduced sound pressure level amonglower frequencies and an increasing sound pressure level among higherfrequencies. The characteristic 70 is achieved from a speakerarrangement having mid to low-frequency speakers placed below a listenerand high-frequency speakers positioned above a listener. Such anarrangement corresponds to an audio system including the speakers 46 and56 of FIG. 2. Having tweeters placed above the ear in the roof of a carprovides an unobstructed and relatively short path directly to anoccupant's ears. Hence, as the curve 70 of FIG. 4D indicates, the soundpressure level at frequencies above 1000 Hz may be significantly higherthan lower frequencies, creating an undesirable audio effect for anearby listener. The sound image resulting from the characteristic curve70 will have a low-frequency component positioned below the occupant 48of FIG. 2, and a high-frequency component positioned above the occupant48.

[0105] The audio characteristics of FIGS. 4B-4D represent various soundpressure levels obtainable within the forward section 42 (shown in FIG.2) and heard by the occupant 48. In an automotive reproductionenvironment having a forward and a rear section, it is possible toreadjust a sound image within each section. Most automobiles areequipped with separate front and rear channels allowing for suchseparate signal correction. The signal conditioning required to correctfor spatial distortion in the rear section 44 will depend on theparticular speaker locations. For example, the speakers 50 of FIG. 2would require substantially the same level of spatial correction as thepair of speakers 46. This is true because the speakers 46 and 50 aresituated in identical positions with respect to a forward occupant 48and a rear occupant, respectively. If however, the rear channel speakersconsist of, or additionally include, the upward facing speakers 52, thena different level of conditioning will be applied, if any, to correctfor spatial distortion in the rear listening compartment of the vehicle40.

[0106] The audio response curves of FIGS. 4B-4D are but a few examplesof how audio signals present at the ears of a listener are distorted byvarious audio reproduction systems. The exact level of spatialdistortion at any given frequency will vary widely depending on thereproduction system and the reproduction environment. Throughapplication of a preferred embodiment of the present invention asdiscussed herein, an apparent location can be generated for a speakersystem defined by apparent elevation and azimuth coordinates, withrespect to a fixed listener, which are different from those of actualspeaker locations.

[0107]FIG. 5 discloses a detailed block diagram of a preferredembodiment of the present invention. A preferred embodiment comprises astereo image correction circuit 22 which inputs the left and rightstereo signals 26 and 28. The image-correction circuit 22 corrects thedistorted spectral densities of various sound systems by advantageouslydividing the audible frequency spectrum into a first frequencycomponent, containing relatively lower frequencies, and a secondfrequency component, containing relatively higher frequencies. Each ofthe left and right signals 26 and 28 is separately processed throughcorresponding low-frequency correction circuits 80, 82, andhigh-frequency correction circuits 84 and 86. It should be pointed outthat in a preferred embodiment the correction circuits 80 and 82 willoperate in a relatively “low” frequency range of approximately 100 to1000 hertz, while the correction circuits 84 and 86 will operate in arelatively “high” frequency range of approximately 1000 to 10,000 hertz.This is not to be confused with the general audio terminology whereinlow frequencies represent frequencies up to 100 hertz, mid frequenciesrepresent frequencies between 100 to 4 kHz, and high frequenciesrepresent frequencies above 4 kHz.

[0108] By separating the lower and higher frequency components of theinput audio signals, corrections in sound pressure level can be made inone frequency range independent of the other. The correction circuits82, 84, 86, and 88 modify the input signals 26 and 28 to correct forspectral and amplitude distortion of the input signals upon reproductionby speakers. The resultant signals, along with the original inputsignals 26 and 28, are combined at respective summing junctions 90 and92. The corrected left stereo signal, L_(c), and the corrected rightstereo signal, R_(c), are provided along outputs 94 and 96,respectively.

[0109] The corrected stereo signals at outputs 94 and 96 have a flat,i.e., uniform, frequency response appearing at the ears of the occupant48 (shown in FIG. 2). This spatially-corrected response creates anapparent source of sound which, when played through the speakers 46 ofFIG. 2, is seemingly positioned directly in front of the occupant 48.Once the sound source is properly positioned through energy correctionof the audio signal, the stereo enhancement circuit 24 conditions thestereo signals to broaden the stereo image emanating from the apparentsound source. As will be discussed in conjunction with FIGS. 8A and 8B,the stereo image enhancement circuit 24 may require adjustment through astereo orientation device 130 to compensate for the actual location ofthe sound source.

[0110] In a preferred embodiment, the stereo enhancement system 24equalizes the difference signal information present in the left andright stereo signals. The stereo enhancement system 24 disclosed hereinis similar to that disclosed in the copending application Ser. No.08/430,751. Related stereo enhancement systems for use in the presentinvention are also disclosed in U.S. Pat. Nos. 4,748,669 and 4,866,774both issued to Arnold Klayman, one of the same inventors of theinvention disclosed in the present application. The disclosures of U.S.Pat. No. 4,748,669, U.S. Pat. No. 4,866,774, and application Ser. No.08/430,751 are incorporated by reference as though fully set forthherein.

[0111] The signals, L_(c) and R_(c), transmitted along paths 94 and 96are inputted by the enhancement system 24 and fed to a high-pass filter98. The filter 98 may in actuality comprise two individual high-passfilters. The filter 98 is a pre-conditioning filter which is designed toreduce the bass components below approximately 100 hertz which may beundesirably present in the difference signal. The outputs from thefilter 98 are transmitted to a difference-signal generator 100. Adifference signal (L_(c)−R_(c)) representing the stereo content of thecorrected left and right input signals, is presented at an output 102.The outputs from the stereo image correction circuit 22 are alsotransmitted directly to a sum signal generator 104. A sum signal,(L_(c)+R_(c)) representing the sum of the corrected left and rightstereo signals is generated at an output 106.

[0112] The sum and difference signals at outputs 102 and 106 are fed toseparate level-adjusting devices 108 and 110, respectively. The devices108 and 110 are ideally potentiometers or similar variable-impedancedevices. Adjustment of the devices 108 and 110 is typically performedmanually to control the base level of sum and difference signal presentin the output signals. This allows a user to tailor the level and aspectof stereo enhancement according to the type of sound reproduced, anddepending on the user's personal preferences. An increase in the baselevel of the sum signal emphasizes the audio information at a centerstage positioned between a pair of speakers. Conversely, an increase inthe base level of difference signal emphasizes the ambient soundinformation creating the perception of a wider sound image. In someaudio arrangements where the music type and system configurationparameters are known, or where manual adjustment is not practical, theadjustment devices 108 and 110 may be eliminated requiring the sum anddifference-signal levels to be predetermined and fixed.

[0113] The output of the device 110 is fed into a stereo enhancementequalizer 120 at an input 122. The equalizer 120 spectrally shapes thedifference signal appearing at the input 122 by separately applying alow-pass audio filter 124 and a high-pass audio filter 126 to thedifference signal. In addition to the conditioning provided by filters124 and 126, the difference-signal level is separately adjusted by astereo orientation circuit 130. Output signals from the filters 124,126, and the orientation circuit 130 exit the equalizer 120 along paths132, 134, and 136, respectively.

[0114] The modified difference signals transferred along paths 132, 134,and 136 are the components of a processed difference signal,(L_(c)−R_(c))_(p), appearing along an output 140. The processeddifference signal is fed into a mixer 142, which also receives the sumsignal from the device 106, as well as the stereo signals L_(c) andR_(c) from outputs 94 and 96. All of these signals are combined withinthe mixer 142 to produce an enhanced and spatially-corrected left outputsignal 30 and right output signal 32.

[0115] The conditioning of the left and right output signals 30 and 32provided by the enhancement circuit 24 is represented by the followingmathematical formulas:

L _(out) =L _(c) +K ₁(L _(c) +R _(c))+K ₂(L _(c) −R _(c))_(p)  (1)

R _(out) =R _(c) +K ₁(L _(c) +R _(c))−K ₂(L _(c) −R _(c))_(p)  (2)

[0116] Although the input signals L_(c) and R_(c) in the equations aboveideally represent corrected stereo source signals, they may also besynthetically generated from a monophonic source. One such method ofstereo synthesis which may be used with the present invention isdisclosed in U.S. Pat. No. 4,841,572, also issued to Arnold Klayman andincorporated herein by reference.

[0117] Image Correction Characteristics

[0118]FIGS. 6A-6C are graphical representations of the levels of spatialcorrection provided by “low” and “high”-frequency correction circuits80, 82, 84, 86 in order to obtain a relocated image generated from apair of stereo signals.

[0119] Referring initially to FIG. 6A, possible levels of spatialcorrection provided by the correction circuits 80 and 82 are depicted ascurves having different amplitude-versus-frequency characteristics. Themaximum level of correction, or boost (measured in dB), provided by thecircuits 80 and 82 is represented by a correction curve 150. The curve150 provides an increasing level of boost within a first frequency rangeof approximately 100 Hz and 1000 Hz. At frequencies above 1000 Hz, thelevel of boost is maintained at a fairly constant level. A curve 152represents a near-zero level of correction.

[0120] To those skilled in the art, a typical filter is usuallycharacterized by a pass-band and stop-band of frequencies separated by acutoff frequency. The correction curves, of FIGS. 6A-6C, althoughrepresentative of typical signal filters, can be characterized by apass-band, a stop-band, and a transition band. A filter constructed inaccordance with the characteristics of FIG. 6A has a pass-band aboveapproximately 1000 Hz, a transition-band between approximately 100 and1000 Hz, and a stop-band below approximately 100 Hz. Filters accordingto FIGS. 6B and 6C have pass-bands above approximately 10 kHz,transition-bands between approximately 1 kHz and 10 kHz, and a stop-bandbelow approximately 1 kHz. Because the filters used in accordance with apreferred embodiment are only first-order filters, the frequenciesdefining the pass, stop, and transition bands are only design goals. Theexact characteristic frequencies may vary significantly for a givencircuit.

[0121] As can be seen in FIGS. 6A-6C, spatial correction of an audiosignal by the circuits 80, 82, 84, and 86 is substantially uniformwithin the pass-bands, but is largely frequency-dependent within thetransition bands. The amount of acoustic correction applied to an audiosignal can be varied as a function of frequency through adjustment ofthe stereo image correction circuit 22 which varies the slope of thetransition bands of FIGS. 6A-6C. As a result, frequency-dependentcorrection is applied to a first frequency range between 100 and 1000hertz, and applied to a second frequency range of 1000 to 10,000 hertz.An infinite number of correction curves are possible through independentadjustment of the correction circuits 80, 82, 84 and 86.

[0122] In accordance with a preferred embodiment, spatial correction ofthe higher frequency stereo-signal components occurs betweenapproximately 1000 Hz and 10,000 Hz. Energy correction of these signalcomponents may be positive, i.e., boosted, as depicted in FIG. 6B, ornegative, i.e., attenuated, as depicted in FIG. 6C. The range of boostprovided by the correction circuits 84, 86 is characterized by amaximum-boost curve 160 and a minimum-boost curve 162. Curves 164, 166,and 168 represent still other levels of boost which may be required tospatially correct sound emanating from different sound reproductionsystems.

[0123]FIG. 6C depicts energy-correction curves that are essentially theinverse of those in FIG. 6B. As previously discussed, attenuation ofhigher frequency sound signals may be required in cases where tweetersare mounted above a listener and apart from the corresponding woofers ormid-range speakers. The levels of attenuation obtainable from thecircuits 84 and 86 may vary from a maximum level of attenuation,represented by a curve 170, to a minimum level of attenuation,represented by a curve 172. Intermediate curves 174, 176, and 178represent some of the possible variances therebetween.

[0124] Since the lower frequency and higher frequency correctionfactors, represented by the curves of FIGS. 6A-6C, are added together,there is a wide range of possible spatial correction curves applicablebetween the frequencies of 100 to 10,000 Hz. FIG. 6D is a graphicalrepresentation depicting a range of composite spatial correctioncharacteristics provided by the stereo image correction circuit 22.Specifically, the solid line curve 180 represents a maximum level ofspatial correction comprised of the curve 150 (shown in FIG. 6A) and thecurve 160 (shown in FIG. 6B). Correction of the lower frequencies mayvary from the solid curve 180 through the range designated by θ₁.Similarly, correction of the higher frequencies may vary from the solidcurve 180 through the range designated by θ₂. Accordingly, the amount ofboost applied to the first frequency range of 100 to 1000 hertz variesbetween approximately 0 and 15 dB, while the correction applied to thesecond frequency range of 1000 to 10,000 hertz may vary fromapproximately 30 dB to −15 dB.

[0125] Image Enhancement Characteristics

[0126] Turning now to the stereo image enhancement aspect of the presentinvention, a series of perspective-enhancement, or normalization curves,is graphically represented in FIG. 7. The signal (L_(c)−R_(c))_(p) inequations 1 and 2 above represents the processed difference signal whichhas been spectrally shaped according to the frequency-responsecharacteristics of FIG. 7. These frequency-response characteristics areapplied by the equalizer 120 depicted in FIG. 5 and are partially basedupon HRTF principles.

[0127] In general, selective amplification of the difference signalenhances any ambient or reverberant sound effects which may be presentin the difference signal but which are masked by more intensedirect-field sounds. These ambient sounds are readily perceived in alive sound stage at the appropriate level. In a recorded performance,however, the ambient sounds are attenuated relative to a liveperformance. By boosting the level of difference signal derived from apair of stereo left and right signals, a projected sound image can bebroadened significantly when the image emanates from a pair ofloudspeakers placed in front of a listener.

[0128] The perspective curves 190, 192, 194, 196, and 198 of FIG. 7 aredisplayed as a function of gain against audible frequencies displayed inlog format. The different levels of equalization between the curves ofFIG. 7 are required to account for various audio reproduction systems.Specifically, in a preferred embodiment, the level of difference-signalequalization is a function of the actual placement of speakers relativeto a listener within an audio reproduction system. The curves 190, 192,194, 196, and 198 generally display a frequency contouringcharacteristic similar to that described in detail in the copendingapplication Ser. No. 08/430,751. That is, lower and higherdifference-signal frequencies are boosted relative to a mid-band offrequencies.

[0129] According to a preferred embodiment, the range for theperspective curves of FIG. 7 is defined by a maximum gain ofapproximately 10-15 dB located at approximately 125 to 150 Hz. Themaximum gain values denote a turning point for the curves of FIG. 7whereby the slopes of the curves 190, 192, 194, 196, and 198 change froma positive value to a negative value. Such turning points are labeled aspoints A, B, C, D, and E in FIG. 7. The gain of the perspective curvesdecreases below 125 Hz at a rate of approximately 6 dB per octave. Above125 Hz, the gain of the curves of FIG. 7 also decreases, but at variablerates, towards a minimum-gain turning point of approximately −2 to +10dB. The minimum-gain turning points vary significantly between thecurves 190, 192, 194, 196, and 198. The minimum-gain turning points arelabeled as points A′, B′, C′, D′, and E′, respectively. The frequenciesat which the minimum-gain turning points occur varies from approximately2.1 kHz for curve 190 to approximately 5 kHz for curve 198. The gain ofthe curves 190, 192, 194, 196, and 198 increases above their respectiveminimum-gain frequencies up to approximately 10 kHz. Above 10 kHz, thegain applied by the perspective curves begins to level off. An increasein gain will continue to be applied by all of the curves, however, up toapproximately 20 kHz, i.e., approximately the highest frequency audibleto the human ear.

[0130] The preceding gain and frequency figures are merely designobjectives and the actual figures will likely vary from circuit tocircuit depending on the actual value of components used. Moreover,adjustment of the signal level devices 108 and 110 will affect themaximum and minimum gain values, as well as the gain separation betweenthe maximum-gain frequency and the minimum-gain frequency.

[0131] Equalization of the difference signal in accordance with thecurves of FIG. 7 is intended to boost the difference signal componentsof statistically lower intensity without overemphasizing thehigher-intensity difference signal components. The higher-intensitydifference signal components of a typical stereo signal are found in amid-range of frequencies between approximately 1 to 4 kHz. The human earhas a heightened sensitivity to these same mid-range of frequencies.Accordingly, the enhanced left and right output signals 30 and 32produce a much improved audio effect because ambient sounds areselectively emphasized to fully encompass a listener within a reproducedsound stage.

[0132] Although the overall equalization applied by the perspectivecurves 190, 192, 194, 196, and 198 is accomplished using high-pass andlow-pass filters of the equalizer 120, it is possible to also use aband-rejection filter to provide the same signal conditioning. Also,implementation of the perspective curve by a digital signal processorwill, in most cases, more accurately reflect the design constraintsdiscussed above. For an analog implementation, it is acceptable if thefrequencies corresponding to the maximum and minimum gains vary by plusor minus 20 percent. Such a deviation from the ideal specifications willstill produce the desired stereo enhancement effect, although with lessthan optimum results.

[0133] As can be seen in FIG. 7, difference signal frequencies below 125Hz receive a decreased amount of boost, if any, through the applicationof the perspective curve 70. This decrease is intended to avoidover-amplification of very low, i.e., bass, frequencies. With many audioreproduction systems, amplifying an audio difference signal in thislow-frequency range can create an unpleasurable and unrealistic soundimage having too much bass response. Examples of such audio reproductionsystems include near-field or low-power audio systems, such asmultimedia computer systems, as well as home stereo systems. A largedraw of power in these systems may cause amplifier “clipping” duringperiods of high boost, or it may damage components of the audio circuitincluding the speakers. Limiting the bass response of the differencesignal also helps avoid these problems in most near-field audioenhancement applications. Further acoustic advantages ofdifference-signal equalization are detailed in the copending applicationSer. No. 08/430,751.

[0134] In accordance with a preferred embodiment, the level ofdifference signal equalization in an audio environment having astationary listener is dependent upon the actual speaker types and theirlocations with respect to the listener. The acoustic principlesunderlying this determination can best be described in conjunction withFIGS. 8A and 8B. FIGS. 8A and 8B are intended to show such acousticprinciples with respect to changes in azimuth of a speaker system.

[0135]FIG. 8A depicts a top view of a sound reproduction environmenthaving speakers 200 and 202 placed slightly forward of, and pointedtowards, the sides of a listener 204. The speakers 200 and 202 are alsoplaced below the listener 204 at an elevational position similar to thatof the speakers 46 shown in FIG. 2. Reference planes A and B are alignedwith ears 206, 208 of the listener 204. The planes A and B are parallelto the listener's line-of-sight as shown.

[0136] It is assumed that sound reproduced by the speakers 200 and 202within the audio environment of FIG. 8A will suffer some spectraldistortion and/or amplitude distortion before impinging upon the ears206 and 208. Such distortion may, for example, be represented by thecurve 64 shown in FIG. 4B which when played through the speakers 200 and202 creates a spatially distorted image. By compensating for thespectral distortion through use of the image correction circuit 22, anaudio signal played through the speakers 200 and 202 will convey anapparent sound image to the listener 204. In the example of FIG. 8A, theapparent sound image will have a different elevation than the actualsound source. Further, by applying the image enhancement aspects of thepresent invention, this apparent sound image will be spatially-enhancedto broaden the apparent image. The resulting image will correspond to anenhanced image emanating from speakers 210 and 212 depicted in phantom.

[0137] Enhancement of the apparent sound image is accomplished byselectively equalizing the difference signal, i.e., the gain of thedifference signal will vary with frequency. The curve 190 of FIG. 7represents the desired level of difference-signal equalization withactual speaker locations corresponding to the phantom speakers 210 and212. However, when speakers are pointed inwardly towards a listener,like the speakers 200 and 202 of FIG. 8A, acoustic perceptions aresignificantly altered which requires a modified level ofdifference-signal equalization. Specifically, direct-field soundemanating from the speakers 200 and 202 enters the listener's ears 206and 208 at an angle 0 ₁ with respect to the reference planes A and B. Asthe speakers are placed further forward the angle θ₁ decreases.Referring now to FIG. 8B, a second sound reproduction system is shownhaving a pair of speakers 214 and 216 placed forward and below thelistener 204. In this configuration, direct-field sound emanating fromthe speakers 214 and 216 enters the listener's ears 206 and 208 at anangle of incidence θ₂ which is smaller than θ₁.

[0138] Most speakers can be characterized by an angle of dispersion, orbeaming characteristic, in which sound is radiated. The angle ofdispersion for sounds of a given frequency will decrease as thefrequency increases. As a consequence, the listener 204 begins to falloutside of the normal beaming aspects of the speakers 200 and 202 asthey are moved forward to the locations of FIG. 8B. When this occurs,the listener 204 will gradually lose perception of a mid-range and uppermid-range of frequencies. Moreover, this effect may be magnified withsmall speakers because smaller speakers typically have an angle ofdispersion narrower than larger speakers.

[0139] To compensate for the loss of mid to upper mid-range of audiofrequencies, the gain of the difference signal is correspondinglyboosted in the same frequency range. As the actual position of thespeakers 200 and 202 is moved forward, the mid-range gain compensationmust be increased. Because the perspective equalization curve 190relatively attenuates this same mid band of frequencies, the level ofattenuation is modified to account for the inwardly-projected speakersof FIGS. 8A and 8B. Accordingly, the curve 196 of FIG. 7 may be used tospatially enhance the system of FIG. 8B to generate the apparent sourceof speakers 218 and 220, while the curve 192 may be sufficient tospatially enhance the system of FIG. 8A. By boosting the differencesignal among the mid-range, or upper mid-range frequencies, an apparentsound image can be properly oriented with respect to the listener 204.Proper orientation of the sound image has the apparent effect ofinwardly rotating the speakers 200, 202, 214 and 216 to direct anapparent dispersion beam at the listener 204.

[0140] Stereo Image Correction Circuit

[0141]FIG. 9 is a detailed schematic diagram of a preferred embodimentof the stereo image correction circuit 22. The circuit 22 is separatedinto a left signal correction circuit 230 and a right signal correctioncircuit 232. The left and right correction circuits 230 and 232 areintended to perform the same signal conditioning upon their respectiveinput signals 26 and 28. Accordingly, the specifications for the leftsignal correction circuit 230 should be identical to those of the rightsignal correction circuit 232. For purposes of simplicity, only thecircuit connections and functional operations of the right signalcorrection circuit 232 will be explained.

[0142] The right stereo signal 28 is input by the right signalcorrection circuit 232 and transferred to a variable resistor 234. Thevariable resistor 234, or potentiometer, is ganged to a similar variableresistor 236 in the left signal correction circuit 230. This is toensure that any adjustments made to the right signal correction circuit232, or vice versa, will affect both circuits 230 and 232 equally. Theright stereo signal is also transmitted along a path 238 to a terminal“1” of a switch 240 which, depending upon the position of the switch240, operates as a bypass preventing any equalization of the stereosignal 28.

[0143] From the variable resistor 234, the input signal is connected toa non-inverting input 242 of a first amplifier 244. The inverting input246 is connected to ground via a resistor 248 and is also connected toone end of a feedback resistor 250. An opposite end of the feedbackresistor 250 is connected to an output 252 of the amplifier 244.

[0144] The output 252 is transmitted to three separate locations of thecircuit 232. Specifically, the output 252 is connected to high-passfiltering circuits 258 and 260, and is also connected to a mixingcircuit 264. With respect to circuit 258, the signal from the output 252is transmitted through a capacitor 266 to a non-inverting input 268 ofan amplifier 270. The input 268 is also connected to ground through aresistor 272. An inverting input 272 of the amplifier 270 is connectedto both ground via resistor 274, and connected to an output 280 of theamplifier 270 through a feedback resistor 276. The filtering circuit 260is configured similarly to circuit 258 with components 284, 286, 288,290, 292, and 294.

[0145] The output 280 and a corresponding output 294 of the amplifier288 are fed to a pair of variable resistors 282 and 296, respectively.The resistor 282 is ganged with a variable resistor 298 of the leftsignal correction circuit 230, while the variable resistor 296 issimilarly ganged with a variable resistor 300. Each of the resistors 282and 296 has a respective output 302 and 304.

[0146] The mixing circuit 264 comprises an amplifier 306 having anon-inverting input 308 connected to ground. Signals provided at theoutputs 302, 304, and 252 enter the mixing circuit 264 and aretransmitted to an inverting input 310 of the amplifier 306. Resistors312, 314, and 316 are connected between the inverting input 310 and theoutputs 252, 302, and 304, respectively. In addition, the signal at theoutput 302 is transmitted to the amplifier 306 via a switch 318 whichmay be a two-position manual or automatic selection switch. A feedbackresistor 320 connects the inverting input 310 to an output 322 of theamplifier 308.

[0147] An amplifier 324 has a non-inverting input 326 connected toground potential, and an inverting input 328 connected to the output322. In addition, the inverting input is connected to the switch 318through a resistor 330. The signal at output 302 is transmitted to theinput 328 via the resistor 330 when the switch 318 is in position 2. Aresistor 332 and a capacitor 334 are connected in parallel between theinput 328 and an output 336 of the amplifier 324. The output 336 is inturn connected to a position “2” of the switch 240. The switch 240 ispreferably ganged with a similar bypass switch 338.

[0148] The left and right energy-correction circuits 230 and 232 modifyamplitude components of the input stereo signals to generate anenergy-corrected left stereo signal 340 and an energy-corrected rightstereo signal 342. Again, for simplicity, reference will only be made togeneration of the energy-corrected right signal 342. It can be easilyappreciated, however, that the same principles apply to generation ofthe energy-corrected left signal 340.

[0149] In operation, the stereo signal 28 is input and processed by thecircuit 232 to correct for distorted sound pressure levels created whenthe signal 28 is played through an audio reproduction system. Initially,the variable resistor 234 allows for adjustment of the input signallevel. Such an adjustment may be required to control the overall gain ofthe circuit 232, or in some instances to boost the input signal 28 if ithas been attenuated significantly by a preceding circuit. The resistor234 may be a standard 10 kohm clockwise potentiometer which is gangedwith the variable resistor 236.

[0150] The amplifier 244 is configured as a voltage follower to act asan isolation buffer between the input signal 28 and the rest of thecircuit 232. The buffered level-adjusted signal appearing at the output252 is fed to the circuit 258 where the signal is passed through asingle-order high-pass filter having a corner frequency of approximately5 kHz. In a preferred embodiment, the high-pass filtering isaccomplished by the RC combination of the capacitor 266 having animpedance of 3900 picofarads and the resistor 272 having an impedance of10 kohms. The resultant high-pass filtered signal appearing at the input268 is buffered through the amplifier 270 operating at unity gain. Theamplitude of the signal appearing at the output 280 may then beincreased or decreased by adjusting the variable resistor 282accordingly.

[0151] Similarly, the circuit 260 inputs the signal from the output 252and processes the signal through the RC filter combination of thecapacitor 284 and the resistor 286. The series connection of thecapacitor 284 and the resistor 286 also operates as a high-pass filterbut with a corner frequency of approximately 500 Hz. This is obtained bychoosing an impedance of 0.022 microfarads for the capacitor 284 and aresistance of 10 kohms for the resistor 286. The filtered input signalis then buffered by the amplifier 288 and fed to the level-adjustingvariable resistor 296.

[0152] The filtered signals exiting the resistors 282 and 296 are fed tothe mixing circuit 264 along paths 302 and 304, respectively. Inaddition, the original signal 28, affected only by a gain adjustment, isalso transferred to the mixer 264 via the resistor 312. The mixercombines all three inputted signals to generate an energy-correctedoutput signal.

[0153] Various levels of spatial correction, as represented in FIGS.6A-6C, are obtained through adjustment of the ganged resistor pairs296/300 and 282/298. Specifically, the lower frequency correction curvesof FIG. 6A can be obtained by measuring the gain at the output 336 whilethe variable resistor 282, which affects higher frequency correction, isset to a minimum. In this setting, the switch 318 will be grounded andno correction of the higher frequencies will occur. Accordingly, therange of lower frequency correction is achieved by adjusting theresistor 296. In this manner, the inverting amplifier 306 combines thefiltered signal at the path 304 with the original signal from the output52. The curve 152 of FIG. 6A represents unity gain whereby the circuit232 merely passes the input signal 28 without any spatial correction.This results when the resistor 296 is set at zero impedance therebygrounding the input 310 of the amplifier 306. As the level of theresistor 296 is increased, more of the filtered signal is added to theoriginal signal providing spatial correction in the 100 to 1000 Hzrange. When the resistor 296 is set at maximum resistance, full spatialcorrection among the lower frequencies is achieved as evidenced by thecurve 150 of FIG. 6A.

[0154] The curves of FIG. 6B represent those obtained by eliminating anycorrection applied by the circuit 260, i.e., setting the resistor 296 tozero impedance, and maintaining the switch 318 in position 1 as shown.Adjustment of the variable resistor 282 provides the desired boost amonghigher frequencies as graphically represented in FIG. 6B. Conversely,attenuation of the higher frequencies, as graphically represented byFIG. 6C, is obtained by setting the switch 318 in position 2. In thisposition, the output from the filtering circuit 258 is provided to aseparate inverting amplifier 324. The amplifiers 306 and 324 thencombine the filtered signals from the paths 302 and 304 in successiveand inverted stages. Specifically, the signal from path 304 and thesignal from the output 252 are first combined by the amplifier 306. Theresultant signal at the output 322, which is now inverted, is thencombined with the output from the resistor 282.

[0155] When combined by the amplifier 306, the gain of the filteredsignals from the paths 302 and 304, relative to the input signal, isdetermined by the impedance ratio of the feedback resistors 320 and 332with the resistors 314 and 316. For most audio reproduction environmentshaving distorted sound pressure levels, these resistors can be set toprovide a maximum gain ratio of approximately 3:1 for the filteredsignals to the input signal. In a preferred embodiment, the resistors320 and 332 will have an impedance of about 10 kohms while the resistors314 and 316 have an impedance of approximately 3.32 kohms. Throughadjustment of the variable resistors 282 and 296, and through selectionof the switch 318, all of the levels of spatial correction representedin FIG. 6D can be obtained.

[0156] The circuit of FIG. 9 represents only a preferred embodiment of astereo image correction circuit. It can be appreciated by a person ofordinary skill in the art that variations in the design of the circuit22 may be made to account for specific reproduction environments withoutdeparting from the intended scope of the invention. For example, theenergy-correction frequency ranges of 0.1-1 kHz (“low” frequencycorrection) and 1 khz-10 kHz (“high” frequency correction) may be variedthrough selection of RC impedance combinations within the filtercircuits 258 and 260. In some instances it may be desirable to havethree or more such energy-correction frequency ranges. It should also benoted that the capacitor 334 is intended to prevent oscillation in thecircuit 22 which may result from stray capacitance present in a discreteimplementation. The capacitor 334 may not be needed in a PC board orsemiconductor implementation of the circuit 22.

[0157] Stereo Image Enhancement Circuit

[0158]FIG. 10 is a schematic diagram of the stereo image enhancementcircuit 24. The circuit 24 is designed to broaden the stereo image ofthe corrected left and right signals L_(c) and R_(c). In accordance witha preferred embodiment, the energy-corrected left signal 340 is fed to aresistor 350, a resistor 352, and a capacitor 354. The energy-correctedright signal 342 is fed to a capacitor 356 and resistors 358 and 360.

[0159] The resistor 350 is connected to a non-inverting terminal 362 ofan amplifier 366. The same terminal 362 is also connected to theresistor 360 and a resistor 368. The amplifier 366 is configured as asumming amplifier having an inverting terminal 370 connected to groundvia a resistor 372. An output 374 of the amplifier 366 is connected tothe inverting terminal 370 via a feedback resistor 376. A sum signal(L_(c)+R_(c)), representing the sum of the left and right signals 340and 342, is generated at the output 374 and fed to one end of a variableresistor 378 which is grounded at an opposite end. For proper summing ofthe signals 340, 342 by the amplifier 366, the values of resistors 350,360, 368, and 376 in a preferred embodiment are approximately twice thatof the resistor 372.

[0160] A second amplifier 380 is configured as a “difference” amplifier.The amplifier 380 has an inverting terminal 382 connected to a resistor384 which is in turn connected in series to the capacitor 354.Similarly, a positive terminal 386 of the amplifier 380 receives thesignal 340 through the series connection of a resistor 388 and thecapacitor 356. The terminal 386 is also connected to ground via aresistor 390. An output terminal 392 of the amplifier 380 is connectedto the inverting terminal through a feedback resistor 394. The output392 is also connected to a variable resistor 396 which is in turnconnected to ground. Although the amplifier 380 is configured as a“difference” amplifier, its function may be characterized as the summingof the right input signal with the negative left input signal togenerate a difference signal (L_(c)−R_(c)). Accordingly, the amplifiers366 and 380 form a summing network for generating a sum signal and adifference signal, respectively.

[0161] The two series connected RC networks comprising elements 354/384and 356/388, respectively, operate as high-pass filters which attenuatethe very low, or bass, frequencies of the input signals L_(c) and R_(c).These RC filters correspond to the device 98 of FIG. 5. To obtain theproper frequency response according to the equalization curves of FIG.7, the cutoff frequency, w_(c), or −3 dB frequency, for the device 98should be approximately 100 Hz. Accordingly, in a preferred embodiment,the capacitors 354 and 356 will have a capacitance of 0.1 microfarad andthe resistors 384, 388 will have an impedance of approximately 33.2kohms. Then, by choosing values for the feedback resistor 394 and theattenuating resistor 390 such that:

R₁₂₀/R₁₂₈=R₁₁₆/R₁₂₄  (3)

[0162] the output 392 will represent the difference signal amplified bya gain of two. The phase of the signal at the output 392 will actuallybe inverted providing the signal R_(c)−L_(c).

[0163] The particular phase of the difference signal is relevant whendetermining the final makeup of the output signal. As is common in theart, use of the term “difference signal” herein denotes both L_(c)−R_(c)and R_(c)−L_(c), which are merely 180 degrees out-of-phase. As can beappreciated by one of ordinary skill in the art, the amplifier 380 couldbe configured so that the “left” difference signal, L_(c)−R_(c), appearsat the output 392, instead of R_(c)−L_(c), as long as the differencesignals at the left and right outputs remain out-of-phase with respectto each other.

[0164] As a result of the high-pass filtering of the inputs, thedifference signal at the output 392 will have attenuated low-frequencycomponents below approximately 125 Hz decreasing at a rate of 6 dB peroctave. It is possible to filter the low frequency components of thedifference signal within the equalizer 120 (shown in FIG. 5), instead ofusing the filter 98. However, because the filtering capacitors at lowfrequencies must be fairly large, it is preferable to perform thisfiltering at the input stage to avoid loading of the preceding circuit.

[0165] The variable resistors 378 and 396, which may be simplepotentiometers, are adjusted by placement of wiper contacts 400 and 402,respectively. The level of difference signal present in the resultantoutput signals may be controlled by manual, remote, or automaticadjustment of the wiper contact 402. Similarly, the level of sum signalpresent in the enhanced output signals is determined in part by theposition of the wiper contact 400. The setting of the contact 402 isreferred to by the applicant as the “SPACE” control for the resultingsound image, while the setting of the contact 400 is the “CENTER”control.

[0166] The sum signal present at the wiper contact 400 is fed to aninverting input 404 of a third amplifier 406 through a series-connectedresistor 408. The same sum signal at the wiper contact 400 is also fedto an inverting input 410 of a fourth amplifier 412 through a separateseries-connected resistor 414. The amplifier 406 is configured as adifference amplifier with the inverting terminal 404 connected to groundthrough a resistor 416. An output 418 of the amplifier 406 is alsoconnected to the inverting terminal 404 via a feedback resistor 420.

[0167] A positive terminal 422 of the amplifier 406 is a summingjunction for a group of signals along signal paths 426. The terminal 422is also connected to ground via a resistor 424. The level-adjusteddifference signal is transmitted from the wiper contact 402 and splitthrough paths 428, 430, and 432. This results in threeseparately-conditioned difference signals appearing at points A, B, andC, respectively. The signals at points A, B, and C, correspond withthose of the outputs 132, 136, and 134 of FIG. 5, respectively. Theconditioned difference signals at points B and C are transferred to thepositive terminal 422 via fixed resistors 432 and 436 as shown. Theconditioned difference signal at point A is transmitted through avariable resistor 438 to the terminal 422.

[0168] The signal at node B represents a filtered version of thelevel-adjusted difference signal appearing across a capacitor 444 whichis connected to ground. The RC network of the capacitor 444 and aresistor 446 operate as a low-pass filter for the difference signal atthe wiper contact 402. This low-pass filter corresponds with the filter124 of FIG. 5. In accordance with a preferred embodiment, the cutofffrequency of this RC network is approximately 200 Hz. Such a cutofffrequency can be realized if the resistor 446 is 1.5 kohms, thecapacitor 444 is 0.47 microfarads, and the drive resistor 434 is 20kohms.

[0169] At node C, the difference signal is filtered by the RCcombination of a resistor 446, connected between node C and ground, anda capacitor 448 connected between node C and the wiper contact 402. Sucha filter corresponds with the high-pass filter 126 of FIG. 5. Theresultant difference-signal component is fed through the drive resistor436 to the terminal 422 of the amplifier 406. The high-pass filter 126is designed with a cutoff frequency of approximately 7 kHz and a gain,relative to that of node B, of −6 dB. Such a cutoff frequency can berealized if the capacitor 448 has an impedance of 4700 picofarads, andthe resistor 180 has a resistance of 3.74 kohms.

[0170] At point A, the level-adjusted difference signal from the wipercontact 402 is transferred to the resistor 440 without selectiveequalization. Accordingly, the signal at point A is merely attenuatedevenly across all frequencies. The signal at point A is furtherattenuated by the impedance of the variable resistor 438, which isadjusted by movement of an associated wiper contact 442.

[0171] Adjustment of the variable resistor 438, which may be a standard100 kohm potentiometer, varies the level of stereo enhancement tocorrect for speaker orientation with respect to a listener. Bydecreasing the resistance of the variable resistor 438, the base levelof difference signal is increased. This causes a corresponding amplitudeincrease in a mid-range of frequencies to partially overcome attenuationof these frequencies by the filters 124 and 126 (shown in FIG. 5).Referring again to FIG. 7, the perspective equalization curve applied tothe difference signal varies from the curve 190 to the curve 198 as theimpedance of the resistor 438 decreases. In this manner, the level ofselective difference-signal equalization may be partially or almosttotally reduced. That is, amplitude adjustment as a function offrequency will be significantly reduced across a mid band offrequencies. Selection of the appropriate curve is determined inaccordance with acoustic principles discussed above in connection withFIGS. 8A and 8B.

[0172] If the stereo image correction circuit 22 and the stereo imageenhancement circuit 24 are applied in a known reproduction environment,then the variable resistor 438 and the resistor 440 may be replaced by asingle fixed resistor having the desired impedance. In a preferredembodiment, the total resistance of the resistors 438 and 440 will varybetween 20 and 100 kohms to account for most reproduction environments.With such a design, the resistor 424 has an impedance of approximately27.4 kohms.

[0173] The modified difference signals present at circuit locations A,B, and C are also fed into the inverting terminal 410 of the amplifier412 through a variable resistor 450 and a fixed resistor 451 seriescombination, and through fixed resistors 452 and 454, respectively.These modified difference signals, the sum signal and theenergy-corrected right signal 342 are transmitted along a group ofsignal paths 456. The signals from the group 456 are combined at theterminal 410 of the amplifier 412. The amplifier 412 is configured as aninverting amplifier having a positive terminal 458 connected to groundand a feedback resistor 460 connected between the terminal 410 and anoutput 462. The resistance level of the variable resistor 450 isadjusted to the same level as that of the resistor 438. To achieveproper summing of the signals by the inverting amplifier 412, theresistor 452 has an impedance of 20 kohms, and the resistor 454 has animpedance of 44.2 kohms. The exact values of the resistors andcapacitors in the stereo enhancement system 24 may be altered as long asthe proper ratios are maintained to achieve the correct level ofenhancement. Other factors which may affect the value of the passivecomponents are the power requirements of the enhancement system 24 andthe characteristics of the amplifiers 370, 380, 406, and 412.

[0174] The signal at the output 418 of the amplifier 406 is fed througha drive resistor 464 to produce the enhanced left output signal 30.Similarly, the signal at the output 462 of the amplifier 412 travelsthrough a drive resistor 466 to produce the enhanced right output signal32. The drive resistors will typically have an impedance on the order of200 ohms.

[0175] In operation, the difference signal components found at points A,B, and C are recombined at the terminal 422 of the difference amplifier406, and at the terminal 410 of the amplifier 412, to form a processeddifference signal (L_(c)−R_(c))_(p). Ideally, the desired range ofperspective curves for generating (L_(c)−R_(c))_(p) is characterized bya maximum gain at approximately 125 Hz and above 7 kHz, and a minimumgain between approximately 2100 Hz and 5 kHz. The processed differencesignal is also combined with the sum signal and either the left or rightsignal to generate output signals L_(out) and R_(out). The enhanced leftand right output signals can be expressed by the mathematical equations(1) and (2) recited above. The value of K₁ in equations (1) and (2) iscontrolled by the position of the wiper contact 400 and the value of K₂is controlled by the position of the wiper contact 402.

[0176] An alternative embodiment of the stereo image enhancement circuit24 is depicted in FIG. 11. The circuit of FIG. 11 is similar to that ofFIG. 10 and represents another method for selectively equalizing adifference signal generated from a pair of stereo audio signals. Thestereo image enhancement circuit 500 generates sum and differencesignals differently than the circuit 24 of FIG. 10.

[0177] In the circuit 500, the left and right energy-corrected signals340 and 342 are fed into negative inputs of mixing amplifiers 502 and504, respectively. To generate the sum and difference signals, however,the left and right signals 340 and 342 are connected to an invertingterminal 510 of a first amplifier 512 through respective resistors 506and 508. The amplifier 512 is configured as an inverting amplifier witha grounded input 514 and a feedback resistor 516. The sum signal, or inthis case the inverted sum signal −(L_(c)+R_(c)), is generated at anoutput 518. The sum signal is then fed to the remaining circuitry afterbeing level-adjusted by a variable resistor 520. Because the sum signalin the circuit 500 is inverted, it is fed to a non-inverting input 522of the amplifier 504. Accordingly, the amplifier 504 now requires acurrent-balancing resistor 524 placed between the non-inverting input522 and ground potential. Similarly, a current-balancing resistor 526 isplaced between an inverting input 528 and ground potential to achievecorrect summing by the amplifier 504 to generate the output signal 32.

[0178] To generate a difference signal, an inverting summing amplifier530 receives the left input signal and the sum signal at an invertinginput 532. The input signal 340 is passed through a capacitor 534 and aresistor 536 before arriving at the input 532. Similarly, the invertedsum signal at the output 518 is passed through a capacitor 540 and aresistor 542. The RC networks created by components 534/536 andcomponents 540/542 provide the bass frequency filtering of the audiosignal as described in conjunction with a preferred embodiment.

[0179] The amplifier 530 has a grounded non-inverting input 544 and afeedback resistor 546. With this alternate configuration of FIG. 11, adifference signal, R_(c)−L_(c), is generated at an output 548 of theamplifier 530. The difference signal is then adjusted by the variableresistor 560 and fed into the remaining circuitry. Acceptable impedancevalues for the circuit 500 include 100 kohms for the resistors 506, 508,516, and 536, impedance values of 200 kohms for the resistors 542 and546, a capacitance of 0.15 micro-farads for the capacitor 540, and acapacitance of 0.33 micro-farads for the capacitor 534. Except asdescribed above, the remaining circuitry of FIG. 11 is the same as thatdisclosed in FIG. 10.

[0180] The stereo image enhancement system 24 may be constructed withonly four active components, typically operational amplifierscorresponding to amplifiers 366, 380, 406, and 412. These amplifiers arereadily available as a quad package on a single semiconductor chip.Additional components needed to complete the stereo enhancement system24 include only 29 resistors (excluding drive resistors) and 4capacitors. The circuit 500 of FIG. 11 can be manufactured with a quadamplifier, 4 capacitors, and only 28 resistors, including thepotentiometers. The circuits 24 and 500 can be formed as a multi-layersemiconductor substrate, i.e., an integrated circuit package.

[0181] Apart from the embodiments depicted in FIGS. 10 and 11, there areadditional ways to interconnect the same components to obtainperspective enhancement of stereo signals in accordance with the presentinvention. For example, a pair of amplifiers configured as differenceamplifiers may receive the left and right signals, respectively, and mayalso each receive the sum signal. In this manner, the amplifiers wouldgenerate a left difference signal, L_(c)−R_(c), and a right differencesignal, R_(c)−L_(c), respectively.

[0182] The stereo image enhancement provided by the enhancement device24 is uniquely adapted to take advantage of high-quality stereorecordings. Specifically, unlike previous analog tape or vinyl albumrecordings, today's digitally stored sound recordings may containdifference signal, i.e. stereo, information throughout a broaderfrequency spectrum, including the bass frequencies. Excessiveamplification of the difference signal is avoided within thesefrequencies by limiting the amount of difference-signal boost in thebass frequencies.

[0183] However, it may be desirable, depending on the sound reproductionenvironment, to boost bass frequencies of the audio signal to compensatefor any loss of bass frequencies which may occur as a result of soundimage relocation and orientation. FIG. 12 depicts a bass-boost circuit550 for use in an alternative embodiment of the present invention tocompensate for any such reduction in bass response. The bass-boostcircuit 550 operates upon the sum signal where most of the bass, i.e.,very low frequency, information resides.

[0184] The circuit 550 has an input, A, receiving the sum signal throughconnection to the output 374 of the amplifier 366 of FIG. 10. The levelof the sum signal is adjusted by a variable resistor 552 which may be a10 kohm potentiometer. The variable resistor 552 may be used as a manualuser-adjust setting, or if the desired amount of bass boost is known,the resistor 552 may be replaced by the appropriate fixed resistor. Thelevel-adjusted sum signal exiting the resistor 552 is then passedthrough a second-order low-pass filter comprised of the resistors 554,556 and the capacitors 558, 562. The resultant filtered signal appearsat a non-inverting terminal of an operational amplifier 564. Theamplifier 564 is configured as a voltage follower to avoid loading ofthe second-order filter. In a preferred embodiment, the gain of theamplifier 564 is set to a maximum of two through selection ofequal-value resistors 566 and 568 which are connected from the invertingterminal to ground and from the inverting terminal to an output,respectively, creating a feedback loop. In a preferred embodiment, theresistors 554, 566, and 568 are 10 kohm resistors, the resistor 556 is a100 kohm resistor, the capacitor 558 has an impedance of 0.1millifarads, and the capacitor 562 has an impedance of 0.01 millifarads.Selection of the foregoing component values allow for selectiveamplification of bass frequencies below approximately 75 hertz throughadjustment of the resistor 552.

[0185] The output of the amplifier 564 is split into two paths eachcontaining a respective fixed resistor 578 and 580. One path, having anoutput labeled X, is connected to the inverting terminal 404 of theamplifier 406 of FIG. 10. Similarly, the output labeled X′ is connectedto the inverting terminal 410 of the amplifier 412. In operation,further boost of the bass frequencies may be obtained by varying theratio of the resistors 578, 580 to the resistors 420 and 460,respectively. For example, in a preferred embodiment, the value of theresistors 578 and 580 will be one-half those of 420 and 426, thusallowing for a gain of two through the amplifiers 406 and 412 of FIG.10. Accordingly, the total gain of the bass-boost circuit 550 may bevaried through a maximum gain of 4 down to zero gain by adjusting theresistor 552.

[0186] It can be appreciated that a variety of stereo enhancementsystems can be substituted for that of the system 24 as desired. Forexample, an embodiment of the systems disclosed in U.S. Pat. Nos.4,748,669 and 4,866,774 equalize the relative amplitudes of both thedifference and the sum signals in specific frequency bands.

[0187] In addition to automobiles, the present invention is suitable fora wide variety of enclosed or outdoor audio reproduction environmentswhere reproduced sound is spatially distorted from the perception of alistener. The present invention may also be used in those environmentswhich do not have listeners situated in a fixed position.

[0188]FIG. 13 depicts one such outdoor audio reproduction environmenthaving outdoor speakers 570 and 572 which create a spatially distortedstereo image with respect to a listener 574. The speakers 570 and 572may be positioned near ground level, as shown in FIG. 13, or in variousother positions to provide stereo sound to a wide outdoor listeningarea. The positioning of the outdoor speakers 570 and 572 will no doubtbe determined in part by factors other than optimum acoustic response.Such positioning, whether it be near the ground, over-head, or insurrounding foliage, may distort the pressure level of emanating soundover certain frequencies as perceived by listeners. The resultingdistorted sound image can be corrected by application of the stereoimage correction circuit 22, and then enhanced by the stereo imageenhancement circuit 24 in accordance with the principles discussedherein. As a result, an apparent sound image can be created which fallswithin a desired listening range 576.

[0189] Some outdoor speakers, like the speakers 570 and 572 of FIG. 13,are omnidirectional to account for the wide listening area and themobility of the listener 574. In such an audio reproduction environment,there is no need to compensate for a reduction in a mid to uppermid-range of frequencies as discussed in connection with FIGS. 8A and8B. Accordingly, optimum enhancement results are achieved in theenvironment of FIG. 13 by applying the perspective curve 190 of FIG. 7to enhance energy-corrected stereo signals played through the speakers570 and 572.

[0190]FIG. 14 depicts another audio reproduction environment containingan implementation of the acoustic correction apparatus 20. Specifically,an electronic keyboard apparatus 590 is shown having speakers 592 and594 placed below a keyboard 596. To an operator (not shown) situated infront of the electronic keyboard 590, the speakers 592 and 594 arelocated at an acoustically-undesirable position beneath the operator'sears. To correct for spatial distortion which may result from suchplacement of the speakers 592 and 594, the acoustic correction apparatus20 modifies audio signals generated by the electronic keyboard 590. Inaccordance with the principles discussed herein, a relocated apparentsound image may thus be generated as emanating from apparent speakers598 and 600 depicted in phantom. Unlike the environment of FIG. 8B, thelevel of orientation required for the audio reproduction environment ofFIG. 14 will likely be minimal due to the positioning of the speakers592 and 594 towards the operator. Accordingly, the curve 190 of FIG. 7may be suitable to spatially enhance the relocated sound image.

[0191] The entire acoustic correction apparatus 20 disclosed herein maybe readily implemented by either (1) a digital signal processor, (2)with discrete circuit components, (3) as a hybrid circuit structure, or(4) within a semiconductor substrate having terminals for adjustment ofthe appropriate resistors. Adjustments by a user currently include thelevel of low-frequency and high-frequency energy correction, varioussignal-level adjustments including the level of sum and differencesignals, and orientation adjustment.

[0192] Through the foregoing description and accompanying drawings, thepresent invention has been shown to have important advantages overcurrent acoustic correction and stereo enhancement systems. While theabove detailed description has shown, described, and pointed out thefundamental novel features of the invention, it will be understood thatvarious omissions and substitutions and changes in the form and detailsof the device illustrated may be made by those skilled in the art,without departing from the spirit of the invention. Therefore, theinvention should be limited in its scope only by the following claims.

What is claimed is:
 1. An audio enhancement apparatus operative uponleft and right stereo input signals provided by a stereo reproductiondevice for playback through a speaker system having a fixed locationwithin an audio reproduction environment, the enhancement apparatusmodifying the stereo input signals to obtain an improved stereo image bycompensating for acoustic limitations created when the input signals arereproduced by the speaker system within the audio reproductionenvironment, the audio enhancement apparatus comprising: a stereo imagecorrection circuit receiving the left and right stereo input signals andmodifying said input signals using at least a first frequency correctioncircuit within a first frequency range and a second frequency correctioncircuit within a second higher frequency range on each of said inputsignals to generate corresponding energy-corrected left and right stereosignals, wherein the first frequency range is processed independentlyfrom the second higher frequency range, said energy-corrected left andright signals creating a corrected spatial response, said correctedspatial response creating an apparent sound image which relocates theperceived position of said speaker system to an apparent position whenheard by a listener; a stereo image enhancement circuit receiving theenergy-corrected left and right stereo signals and generating enhancedleft and right stereo signals to provide a spatially enhanced apparentsound image which is perceived by said listener to substantially emanatefrom said apparent position when said enhanced left and right stereosignals are reproduced through said speaker system; and wherein saidenergy-corrected left and right signals are characterized by a firstambient component, and said enhanced left and right stereo signals arecharacterized by a second ambient component, said second ambientcomponent selectively equalized with respect to said first ambientcomponent.